Organic light emitting devices (OLEDs) are typically multilayer devices which upon an applied voltage are capable emitting light from the radiative relaxation of an excited state located on an organic material. OLEDs have found widespread application as an alternative to liquid crystal displays (LCDs) for handheld devices or flat panel displays. Furthermore, OLEDs have shown promise as next generation solid state white lighting, use in medical devices, and as infrared emitters for communication applications. The use of organic materials presents a number of unique benefits including: compatibility with flexible substrates, capabilities for large scale production, and simplified tuning of the emission properties through molecular modification.
FIG. 1 depicts a cross-sectional view of an OLED 100. OLED 100 includes substrate 102, anode 104, hole-transporting material(s) (HTL) 106, light processing material (EML) 108, electron-transporting material(s) (ETL) 110, and a metal cathode layer 112. Anode 104 may be indium tin oxide (ITO). Light processing material 108 may be an emissive material (EML) including an emitter and a host. A typical OLED device includes at least one transparent electrode through which light is emitted. For example, OLEDs that emit through the bottom substrate typically contain a transparent conductive oxide material, such as indium tin oxide, as an anode, while at the cathode a reflective metal is typically used. Alternatively, devices may emit from the top through a thin metal layer as the cathode while having an either opaque or transparent anode layer. In this way it is possible to have dual emission from both top and bottom if such a device is so desired and furthermore it is possible for these OLEDs to be transparent. Sandwiched between the electrodes is typically a multilayer organic stack. A multilayer organic stack typically includes a single layer of hole-transporting materials (HTL), a single layer of emissive materials (EML) including emitters and hosts, a single layer of electron-transporting materials (ETL) and a layer of metal cathode, as depicted in FIG. 1.
For each of the transport layers care must be taken to optimize the separate process of facilitating charge injection, having efficient charge transport, and confining the charges and excitons in a specified emissive region (typically the emissive layer). Such a process can be achieved through either a single material or through a multilayer stack which may separate the injection, transport, charge confining, and exciton confining tasks. The emissive layer may be composed of a single emissive material, a single emissive material dispersed in a host matrix material, multiple emissive materials dispersed in a host matrix, or any number of emissive materials dispersed in multiple host materials. The host materials are typically chosen carefully to not quench the excited state of the emitter as well as to provide appropriate distribution of charges and excitons within the emissive layer. The emission color of the OLED is determined by the emission energy (optical energy gap) of emitters.
Light is generated in OLEDs through the formation of excited states from separately injected electrons and holes to form an exciton, located on the organic material. Due to the uncorrelated nature of the injected charges, excitons with total spin of 0 and 1 are possible. Spin 0 excitons are denoted singlets, while spin 1 excitons are denoted triplets, reflecting their respective degeneracies. Due to the selection rules for radiative transitions, the symmetry of the excited state and the ground state must be the same. Since the ground state of most molecules are antisymmetric, radiative relaxation of the symmetric triplet excited state is typically disallowed. As such, emission from the triplet state, called phosphorescence, is slow and the transition probability is low. However emission from the singlet state, called fluorescence, can be rapid and consequently efficient. Nevertheless, statistically there is only 1 singlet exciton for every 3 triplet excitons formed. There are very few fluorescent emitters which exhibit emission from the triplet state at room temperature, so 75% of the generated excitons are wasted in most fluorescent emitters. However, emission from the triplet state can be facilitated through spin orbit coupling which incorporates a heavy metal atom in order to perturb the triplet state and add in some singlet character to and achieve a higher probability of radiative relaxation.
Some efficient emitters include heavy metals such as Ir, Pt, Pd, Au, Os, Rh, and Ru, which can emit efficiently across the visible spectrum. Thus, due to their typically high efficiencies, phosphorescent OLEDs (i.e. OLEDs with phosphorescent materials as emitters) have been a mainstay in OLED development. Recently, OLEDs with electron to photon conversion efficiencies near 100% across the visible spectrum have been demonstrated. However, there remains a deficit of efficient phosphorescent emissive materials that also demonstrate long operational stability in a device setting, particularly in the blue region. Fluorescent OLEDs (i.e., OLEDs with fluorescent materials as emitters), on the other hand, have found widespread use in devices with long operational lifetime. Furthermore, fluorescent emitters typically do not contain precious metals and are not affected by triplet-triplet annihilation which degrades device performance at high current densities.